Sphingosylphosphocholine (SPC) modulates Ca2+ release from isolated cardiac sarcoplasmic reticulum membranes; 50 microM SPC induces the release of 70 80% of the accumulated calcium. SPC release calcium from cardiac sarcoplasmic reticulum through the ryanodine receptor, since the release is inhibited by the ryanodine receptor channel antagonists ryanodine. Ruthenium Red and sphingosine. In intact cardiac myocytes, even in the absence of extracellular calcium. SPC causes a rise in diastolic Ca2+, which is greatly reduced when the sarcoplasmic reticulum is depleted of Ca2+ by prior thapsigargin treatment. SPC action on the ryanodine receptor is Ca(2+)-dependent. SPC shifts to the left the Ca(2+)-dependence of [3H]ryanodine binding, but only at high pCa values, suggesting that SPC might increase the sensitivity to calcium of the Ca(2+)-induced Ca(2+)-release mechanism. At high calcium concentrations (pCa 4.0 or lower), where [3H]ryanodine binding is maximally stimulated, no effect of SPC is observed. We conclude that SPC releases calcium from cardiac sarcoplasmic reticulum membranes by activating the ryanodine receptor and possibly another intracellular Ca(2+)-release channel, the sphingolipid Ca(2+)-release-mediating protein of endoplasmic reticulum (SCaMPER) [Mao, Kim, Almenoff, Rudner, Kearney and Kindman (1996) Proc.Natl.Acad.Sci. U.S.A 93, 1993-1996], which we have identified for the first time in cardiac tissue.

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Calcium release from sarcoplasmic reticulum (SR) has been elicited in response to additions of many different agents. Activators of Ca2+ release are here tentatively classified as activators of a Ca2+-induced Ca2+ release channel preferentially localized in SR terminal or as likely activators of other Ca2+ efflux pathways. Some of these pathways may be associated with several different mechanisms for SR Ca2+ release that have been postulated previously. Studies of various inhibitors of excitation-contraction coupling and of certain forms of SR Ca2+ release are summarized. The sensitivity of isolated SR to certain agents is unusually affected by experimental conditions. These effects can seriously undermine attempts to anticipate effects of the same pharmacological agents in situ. Finally, mention is made of a new preparation ("sarcoballs") designed to make the pharmacological study of SR Ca2+ release more accessible to electrophysiologists, and some concluding speculations on the future of SR pharmacology are offered.

Recent evidence indicates that sphingolipids are produced by the heart during hypoxic stress and by blood platelets during thrombus formation. It is therefore possible that sphingolipids may influence heart cell function by interacting with G-protein-coupled receptors of the Edg family. In the present study, it was found that sphingosine 1-phosphate (Sph1P), the prototypical ligand for Edg receptors, produced calcium overload in rat cardiomyocytes. The cDNA for Edg-1 was cloned from rat cardiomyocytes and, when transfected in an antisense orientation, effectively blocked Edg-1 protein expression and reduced the Sph1P-mediated calcium deregulation. Taken together, these results demonstrate that cardiomyocytes express an extracellular lipid-sensitive receptorsystem that can respond to sphingolipid mediators. Because the major source of Sph1P is from blood platelets, we speculate that Edg-mediated Sph1P negative inotropic and cardiotoxic effects may play important roles in acute myocardial ischemia where Sph1P levels are probably elevated in response to thrombus.

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